Known prior art includes various devices and methods which are suitable for removal of material from the surface of an object by means of laser radiation and accordingly for shaping these objects, for example, for ablating tissue in the corneal region of the eye and for ophthalmologic shaping of eye lenses.
The first publications on influencing visual deficiency of the human eye by changing the convexity or concavity of the cornea date from around 1983 to 1985. Comparatively more corneal tissue must be removed in the center of the eye lens than in the peripheral areas in order to achieve flattening and, as a result, a correction of nearsightedness. However, if more corneal tissue is removed at the periphery than in the center, the curvature of the cornea is increased in order to correct farsightedness.
As a result, different amounts of biological substance are to be removed depending on the indication of individual surface portions of the cornea. In addition, the quantity of substance to be removed per time unit can vary depending on the extent of the correction required and depending on the progress of treatment; for example, a larger amount is to be removed in the first stage of treatment than in the concluding stage of precision treatment during which the main concern is to achieve smooth surfaces on the corrected curvature.
An essential factor for the quantity removed per time unit and accordingly also for a changeable defined rate of removal is, for one, the intensity of laser radiation itself, i.e., the energy introduced into the material to be removed by the radiation, and also the intensity distribution over the cross section of the laser radiation or in the spot applied to the surface of the object with every laser pulse. If the intensity distribution in the radiation cross section varies, the amount removed over the cross-sectional area will also vary.
Differing amounts to be removed across the cross-sectional area is desirable, for instance, when less material is to be ablated at the edges of the cross section or spot than in a central radiation area because, in this way, the formation of steep edge regions in the remaining material can be avoided.
The radiation emanating from an excimer laser has a rectangular cross section in which, intensity fluctuations aside, a more uniform intensity distribution is given in the direction of the greater cross-sectional length than in the direction of the shorter cross-sectional side oriented at right angles to the first direction, where the intensity falls in a bell-shaped or Gaussian shape from the center of radiation to the edges. Elaborate steps are required in order to homogenize the radiation in a cross-sectional direction or also within the entire cross section. Homogenizing by means of scattering plates followed by diaphragms and through the use of abrasive diaphragms is known, for example.
Devices for homogenizing radiation intensity especially in excimer laser radiation are described, for example, in DE 42 20 705, JP 07027993, EP 0 232 037, and EP 0 100 242. The arrangements shown in these references serve to distribute the radiation intensity as uniformly as possible over the entire radiation cross section. However, a uniform intensity over the entire cross section would mean a pot-shaped intensity distribution, namely, that the intensity rises or falls very steeply in the edge regions of the laser beam. If laser radiation of this kind is guided over the surface of the object to be treated according to the spot scanning principle, the pot-shaped intensity distribution results in a step formation in the remaining material in the boundary areas from spot to spot. Steplike irregularities of this kind on the cornea lead to troublesome optical phenomena in sensory perception.
OS-DE 44 29 193 A1 describes another device for generating a cross-sectionally homogenized laser beam and the use of this radiation for removal of material. In this case, a pulsed laser radiation emanating from a solid state laser is guided through an optical fiber and mode-homogenized in this way. It is disadvantageous that the arrangement described in this reference is not suitable for spot scanning, i.e., it is possible to treat only relatively large surface portions (spots) in their entirety.
References to whole-surface ablation of the cornea with a solid state laser with Gaussian intensity distribution in the radiation cross section are contained in the article “Fundamental Mode Photoablation of the Cornea for Myoptic Correction,” T. Sailer and J. Wollensack, Laser and Light in Ophthalmology, Vol. 5, No. 4, pp 199-203, 1993. The procedure described therein assumes that a laser of this kind delivers a spatially homogeneous radiation in the fundamental mode TEM00. However, only a portion of the radiated energy is available in the fundamental mode TEM00, this portion not being sufficient, for example, for corneal ablation.